hack/build-image: use constants already defined in the code

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

hack/build-image

@@ -463,9 +463,9 @@ def add_special_tags(img, distro_qualified=True):
             img.additional_tags.append((NIGHTLY, QUAL_NONE))
     if not distro_qualified:
         return  # skip creating "distro qualified" tags
-    if img.arch == host_arch() and img.pkg_source == "default":
+    if img.arch == host_arch() and img.pkg_source == DEFAULT:
         img.additional_tags.append((f"{img.distro}-{LATEST}", QUAL_DISTRO))
-    if img.arch == host_arch() and img.pkg_source == "nightly":
+    if img.arch == host_arch() and img.pkg_source == NIGHTLY:
         img.additional_tags.append((f"{img.distro}-{NIGHTLY}", QUAL_DISTRO))